This rule is deprecated; use {rule:javascript:S2201} instead.

Why is this an issue?

Doing an operation on a string without using the result of the operation is useless and is certainly due to a misunderstanding.

Noncompliant code example

var str = "..."
str.toUpperCase(); // Noncompliant

Compliant solution

var str = "..."
str = str.toUpperCase();